Push Notifications

This document describes how to use push notifications that inform your application when a resource changes.

Overview

The Google Calendar API provides push notifications that let you watch for changes to resources. You can use this feature to improve the performance of your application. It allows you to eliminate the extra network and compute costs involved with polling resources to determine if they have changed. Whenever a watched resource changes, the Google Calendar API notifies your application.

To use push notifications, you need to do three things:

Register the domain of your receiving URL.

For example, if you plan to use https://mydomain.com/notifications as your receiving URL, you need to register https://mydomain.com.

Set up your receiving URL, or "Webhook" callback receiver.

This is an HTTPS server that handles the API notification messages that are triggered when a resource changes.

Set up a notification channel for each resource endpoint you want to watch.

A channel specifies routing information for notification messages. As part of the channel setup, you identify the specific URL where you want to receive notifications. Whenever a channel's resource changes, the Google Calendar API sends a notification message as a POST request to that URL.

Registering your domain

Before you can set up a push notification channel, you must register the domain for any URLs you
plan to use to receive push notification messages. In addition, before you register a domain, you
must first verify that you own it. This step is an abuse-prevention measure to stop anyone from
using push to send messages to someone else’s domain.

Step 1: Verify that you own the domain

Before you can register your domain, you need to verify that you own it. Complete the site
verification process using Search Console.
For more details, see the site
verification help documentation.

Step 2: Register your domain

To register a verified domain name as one of the allowed domains for your project, do the
following:

At this point, the Google API Console checks all domains in the list against the ones that you
have verified in Search Console. Assuming
that you properly verified all the domains, the page updates to show your new list of allowed
domains. You now can use any of these domains to receive push notifications.

Creating notification channels

To request push notifications, you need to set up a notification channel for each resource you want to watch. After your notification channels are set up, the Google Calendar API will inform your application when any watched resource changes.

Making watch requests

Each watchable Google Calendar API resource has an associated watch method at a URI of the following form:

https://www.googleapis.com/apiName/apiVersion/resourcePath/watch

To set up a notification channel for messages about changes to a particular resource, send a POST request to the watch method for the resource.

Each notification channel is associated both with a particular user and a particular resource (or set of resources). A watch request will not be successful unless the current user
owns or has permission to access this resource.

Example

Start watching for changes to a collection of events on a given calendar:

Required properties

With each watch request, you need to provide:

An id property string that uniquely identifies this new notification channel within your project. We recommend that you use a universally unique identifier (UUID) or any similar unique string. Maximum length: 64 characters.

The ID value you set is echoed back in the X-Goog-Channel-Id HTTP header of every notification message that you receive for this channel.

A type property string set to the value web_hook.

An address property string set to the URL that listens and responds to notifications for this notification channel. This is your Webhook callback URL, and it must use HTTPS.

Note that the Google Calendar API will be able to send notifications to this HTTPS address only if there is a valid SSL certificate installed on your web server. Invalid certificates include:

Self-signed certificates.

Certificates signed by an untrusted source.

Certificates that have been revoked.

Certificates that have a subject that doesn't match the target hostname.

Optional properties

You can also specify these optional fields with your watch request:

A token property that specifies an arbitrary string value to use as a channel token. You can use notification channel tokens for any of a variety of purposes. For example, you can use the token to verify that each incoming message is for a channel that your application created—to ensure that the notification is not being spoofed—or to route the message to the right destination within your application based on the purpose of this channel. Maximum length: 256 characters.

The token is included in the X-Goog-Channel-Token HTTP header in every notification message that your application receives for this channel.

If you use notification channel tokens, we recommend that you:

Use an extensible encoding format, such as URL query parameters. Example: forwardTo=hr&createdBy=mobile

Do not include sensitive data such as OAuth tokens.

Note: If you do need to send highly sensitive data, make sure it is encrypted before adding it to the token.

An expiration property string set to a Unix timestamp (in ms) of the date and time when you want the Google Calendar API to stop sending messages for this notification channel.

If a channel has an expiration time, it is included as the value of the X-Goog-Channel-Expiration HTTP header (this time in human-readable format) in every notification message that your application receives for this channel.

In addition to the properties you sent as part of your request, the returned information also includes the resourceId and resourceUri to identify the resource being watched on this notification channel.

Note: The resourceId property is a stable, version-independent identifier for the resource. The resourceUri property is the canonical URI of the watched resource in the context of the current API version, so it’s version-specific.

Sync message

After creating a new notification channel to watch a resource, the Google Calendar API sends a sync message to indicate that notifications are starting. The X-Goog-Resource-State HTTP header value for these messages is sync. Because of network timing issues, it is possible to receive the sync message even before you receive the watch method response.

It is safe to ignore the sync notification, but you can also make use of it. For example, if you decide you do not want to keep the channel, you can use the X-Goog-Channel-ID and X-Goog-Resource-ID values in a call to stop receiving notifications. You can also use the sync notification to do some initialization to prepare for later events.

Sync messages always have an X-Goog-Message-Number HTTP header value of 1. Each subsequent notification for this channel will have a message number that is larger than the previous one, though the message numbers will not be sequential.

Renewing notification channels

A notification channel can have an expiration time, with a value determined either by your request or by any Google Calendar API internal limits or defaults (the more restrictive value is used). The channel's expiration time, if it has one, is included as a Unix timestamp (in ms) in the information returned by the watch method. In addition, the expiration date and time is included (in human-readable format) in every notification message your application receives for this channel in the X-Goog-Channel-Expiration HTTP header.

Currently there is no automatic way to renew a notification channel. When a channel is close to its expiration, you must create a new one by calling the watch method. As always, you must use a unique value for the id property of the new channel. Note that there is likely to be an "overlap" period of time when the two notification channels for the same resource are active.

Receiving notifications

Whenever a watched resource changes, your application will receive a notification message describing the change. The Google Calendar API sends these messages as HTTPS POST requests to the URL you specified as the "address" for this notification channel.

Note: These notification delivery HTTPS requests specify a user agent of APIs-Google and respect robots.txt directives, as described in APIs Google User Agent.

Understanding the notification message format

All notification messages include a set of HTTP headers that have X-Goog- prefixes.
Some types of notifications can also include a message body.

Headers

Notification messages posted by the Google Calendar API to your receiving URL include the following HTTP headers:

Header

Description

Always present

X-Goog-Channel-ID

UUID or other unique string you provided to identify this notification channel.

X-Goog-Message-Number

Integer that identifies this message for this notification channel. Value is always 1 for sync messages. Message numbers increase for each subsequent message on the channel, but they are not sequential.

X-Goog-Resource-ID

An opaque value that identifies the watched resource. This ID is stable across API versions.

X-Goog-Resource-State

The new resource state, which triggered the notification. Possible values:
sync, exists, or not_exists.

X-Goog-Resource-URI

An API-version-specific identifier for the watched resource.

Sometimes present

X-Goog-Channel-Expiration

Date and time of notification channel expiration, expressed in human-readable format. Only present if defined.

X-Goog-Channel-Token

Notification channel token that was set by your application, and that you can use to verify the source of notification. Only present if defined.

Notification messages posted by the Google Calendar API to your receiving URL do not include a message body.

Responding to notifications

To indicate success, you can return any of the following status codes: 200, 201, 202, 204, or 102. If your service returns 500, 502, 503, or 504, the Google Calendar API will retry with exponential backoff.

Every other return status code is considered to be a message failure and the Google Calendar API will not retry this particular notification.

Understanding Google Calendar API notification events

This section provides details on the notification messages you can receive when using push notifications with the Google Calendar API.

X-Goog-Resource-State

Applies to

Delivered when

sync

ACLs, Calendar lists, Events, Settings.

A new channel was successfully created. You can expect to start receiving notifications for it.

exists

ACLs, Calendar lists, Events, Settings.

There was a change to a resource. Possible changes include the creation of a new resource, or the modification or deletion of an existing resource.

Stopping notifications

The expiration time is when the notifications stop automatically. You can choose to stop receiving notifications for a particular channel before it expires by calling the stop method at
the following URI:

https://www.googleapis.com/calendar/v3/channels/stop

This method requires that you provide at least the channel’s id and the resourceId properties, as shown in the example below. Note that even if the Google Calendar API has several types of resources that have watch methods, there is only one stop method.

Only users with the right permission can stop a channel. In particular:

If the channel was created by a regular user account, only the same user from the same client (as identifed by the OAuth 2.0 client IDs from the auth tokens) who created the channel can stop the channel.

If the channel was created by a service account, any users from the same client can stop the channel.

Special Considerations

Events and ACLs are per-calendar If you want to get notified about all event or ACL changes for calendars A and B, you need to separately subscribe to the events/ACL collections for A and for B.

Settings and calendar lists are per-user Settings and calendar lists only have one collection per user, so you can subscribe just once.

Also, you won’t be notified when you gain access to a new collection (for example, a new calendar) although you will be notified if that calendar is added to the calendar list (assuming you are subscribed to the calendar list collection).

Notifications are not 100% reliable. Expect a small percentage of messages to get dropped under normal working conditions. Make sure to handle these
missing messages gracefully, so that the application still syncs even if no push messages are received.